Ecological succession | Biodiversity and human impacts | High school biology | Khan Academy

You look at a community that is in a given habitat. A natural question is to say, "Well, has that community always been that way? Has it always been there? Was there a time where maybe there was no life there?" And the answer is, well, yes, the communities do change over time. There is some initial period where there might not be any life in that habitat, and then life slowly colonizes it. The makeup of that life will change over time, the makeup of that community.

This general idea is called ecological succession. Ecological succession, and folks will often talk about the different types of ecological succession, splitting it up into primary ecological succession. Primary is when you start with no life because you really have a new habitat, and then slowly life colonizes it.

The best example of that—or one of the best examples of that—is when new land forms due to lava flows. These are pictures from Hawaii, where new land is forming as this lava hardens. At first, when it's molten lava, there's no life there. Then it hardens, and slowly basic life, or life in general, will start to colonize that lava rock. Some of it you won't be able to see with your naked eye; it would be microbes. Some of it you could see; it could be simple ferns and plants like that. These are often called the pioneer species.

But what they often do is make that environment more suitable for other types of life. They might slowly break down that rock as they die. Along with the broken-down rock, that also gets eroded from the water, the air, and the rain. It starts to make soil and conditions more suitable for other types of species.

These pioneer species, they don't even have to just be plants and microbes. I was just reading an article about how in Hawaii, humans want to get that land because it's beachfront property. The beach might not have formed in the traditional sense, but you have ocean view property. So humans might be some of the first pioneer species who might want to be out on that land.

That new land doesn't just form from lava flows; there are other examples of new habitats forming. So right here, we have pictures of a new habitat forming because of the retreat of glaciers. When the glaciers were covering up these rocks, you didn't have life on them. But as the glaciers— as the glacier retreats, right over here, you see things like these mosses and other types of pioneer species starting to colonize. Over time, they're going to make it more and more suitable for other types of species.

So that’s primary succession. Another situation is when you have secondary succession. There are many different ways you could have secondary succession. One of the often cited examples is when you have some type of a disaster.

So this right over here, this is a picture of a fire. Here we're talking about secondary succession, where you had a community but then you have a fire. That fire might wipe out a lot of the community, and then it creates space for other things to form. So after you have a fire, the forest might look something like this.

Then notice you have more species that start to colonize where a lot of other species might have died during the fire. Sometimes, after this disaster of some kind, you might get back to the same type of community that you had before the disaster. But sometimes it could be a completely different one— that the communities don’t come about in exactly the same way.

So the general idea is communities change over time. We have ecological succession. There are times when there's no community, and then they come in— that's primary succession. Then you have times where you have disasters of some kind that could change the environment in some ways, and it could change the makeup of that community.

Things don't have to be as dramatic as new land formation because of lava, or because of forest fires, or even glaciers retreating. It could be because of a disease, or it could just be because a new species gets introduced somehow that changes the makeup— changes the competition, the predatory dynamics, the various symbiotic dynamics within that community.